1. Field of the Invention
The present invention relates to an image processing method and an image processing program for visualizing a tubular tissue.
2. Description of the Related Art
A technique for visualizing the inside of a three-dimensional object has attracted public attention with the advance of image processing technology using a computer in recent years. Particularly in the medical field, medical diagnosis using a CT (Computed Tomography) apparatus or MRI (Magnetic Resonance Imaging) apparatus has been performed widely because a lesion can be detected early by visualizing the inside of a living body.
On the other hand, volume rendering is known as a method for obtaining a three-dimensional image of the inside of an object. In volume rendering, ray is emitted onto a three-dimensional voxel (micro volume element) space to thereby project an image on a projection plane. This operation is referred to as ray casing. In ray casting, a voxel value is acquired from a voxel at each sampling point which is sampled at a regular interval along the path of the ray.
The voxel is a unit for constituting a three-dimensional region of an object. The voxel value is a specific data expressing characteristic such as a density value of the voxel. The whole object is expressed by voxel data which is a three-dimensional arrangement of the voxel value. Generally, two-dimensional tomogram data obtained by CT is collected along a direction perpendicular to each sectional layer, and voxel data which is the three-dimensional arrangement of voxel value is obtained by performing necessary interpolation.
In ray casting, virtual reflected light of a virtual ray emitted onto an object from a virtual viewpoint is generated according to an opacity value artificially set for each voxel value. Then, the gradient of voxel data, that is, a normal vector is obtained to obtain a virtual surface, and a shading coefficient for shading is calculated from the cosine of an angle between the virtual ray and the normal vector. Virtual reflected light is calculated by multiplying the intensity of the virtual ray emitted on each voxel, the opacity value of the voxel and the shading coefficient.
FIG. 12A shows an example of a colon being displayed by a parallel projection method of volume rendering as an example of visualization of a tubular tissue in the inside of a human body. According to such volume rendering, a see-through image of the three-dimensional structure of the colon can be formed from two-dimensional tomogram data obtained successively along a direction perpendicular to sectional layers of the abdomen. The image obtained by the parallel projection method is suitable for observation from the outside but unsuitable for observation from the inside.
FIG. 12B shows an example of achieving an image obtained by a virtual endoscope by generating a centrally projected image of the inside of the colon with volume rendering. When voxel data is reconstructed from a viewpoint in the inside of the tubular tissue in this manner, inspection with an endoscope can be simulated. Accordingly, a polyp or the like in the inside of the tubular tissue can be detected.
However, the virtual endoscope image has a disadvantage that a large number of images obtained by the virtual endoscope has to be referred to perform diagnosis because the region allowed to be displayed at one time in each image obtained by the virtual endoscope is small.
FIGS. 13A and 13B are views for explaining a parallel projection method and a central projection method respectively. In the parallel projection method, as shown in FIG. 13A, virtual ray 82 is emitted parallel from a virtual viewpoint 81, and an image can be generated to observe an observation target 83 mainly from the outside. On the other hand, in the central projection method, as shown in FIG. 13B, virtual ray 85 is emitted radially from a virtual viewpoint 84. In the central projection method, an image with perspective and reality as the human sees an observation target 86 with his eyes can be generated.
FIGS. 14A and 14B show an example of display of an exfoliated image of a tubular tissue using a cylindrical coordinate system in ray casting. According to the central projection method shown in FIG. 13B, inspection of the colon or the like with an endoscope can be simulated, but it is difficult to understand the position or size of a polyp or the like in the wall of the tubular tissue accurately when the inside of the colon is inspected while scanned.
Therefore, as shown in FIG. 14A, a virtual viewpoint 91 is placed on a center line 94 of a colon 93. Virtual ray 92 is radiated from the virtual viewpoint 91 in directions perpendicular to the center line 94, and an image of the inner wall surface of the colon 93 is generated. Then, the image is cut open in parallel to the center line 94 so that an exfoliated image of the inner wall surface of the colon can be displayed as shown in FIG. 14B.
FIGS. 15A to 15E are views for explaining a cylindrical projection method using a cylindrical coordinate system. FIG. 15A shows a cylindrical coordinate system 102 set in the inside of a tubular tissue 101 and a virtual ray 103 radiated from the center axis of the cylindrical coordinate system 102. FIG. 15B shows a state in which the cylindrical coordinate system 102 is represented as C(h,α) based on a distance h along the center axis and an angle α around the center axis. FIG. 15C shows a state in which the cylindrical coordinate C(h,α) is exfoliated and converted into two-dimensional coordinates l(u,v). Each of FIGS. 15D and 15E shows a state in which the virtual ray 103 is radiated from the center axis of the tubular tissue 101. Accordingly, by assuming that a cylindrical coordinate system 102 is set virtually in the inside of a tubular tissue 101 and performing the projection radially from the center axis of the cylindrical coordinate system 102 in this manner, a 360° panoramic image of the inner wall surface of the tubular tissue 101 can be generated.
FIGS. 16A and 16B are views for explaining a curved cylindrical projection method when a tubular tissue as a subject of observation is curved. As shown in FIGS. 16A and 16B, the curved cylindrical projection method is a method of projection in which virtual ray 113 is radiated from a curved center line 112 when the tubular tissue 111 as a subject of observation is curved. As described above, in accordance with the curved cylindrical projection method, by assuming the central path 112 along the real curved internal organ of the human body, and by performing projection with the central path 112 as the center, inspection can be performed with CT data (for example, see “Virtual Colon Unfolding”, A. Vilanova Bartroli, R. Wegenkittl, A. Konig and E. Groller, IEEE Visualization, U.S., pp. 411-420, 2001).
In related arts, there is a problem in curved cylindrical projection method when the curve is sharp. When the curve is sharp, virtual rays intersect each other during rendering process, resulting in that some regions of the inner wall surface of the tubular tissue may appear multiple times on the projected image, while other region may not appear at all. Some related arts aim to solve this problem (see “Virtual Colon Unfolding”, A. Vilanova Bartroli, R. Wegenkittl, A. Konig and E. Groller, IEEE Visualization, U.S., pp. 411-420, 2001). In order to avoid such a problem, several methods are proposed where virtual rays progress on a curved surface or an oblique plane surface so as not to intersect with each other. However, the curved or plane surface is mathematically differentiable (smooth) at nearby the intersection point of the surface and the central path. Because of this constraint, the virtual rays hardly reach the backside of complex folds.
The above problem cannot be solved by the curved cylindrical projection method in the related art, even when virtual ray is not linear or perpendicular to the central path. In related art, several methods are proposed where virtual ray progress on a curved surface or an oblique plane surface. Those methods are designed to avoid ray intersections. In the curved cylindrical projection method with linear ray casting, virtual rays may intersect with each other at area where the tubular tissue has a large curvature.
Another problem in the curved cylindrical projection method according to the related art is that a region which can be hardly observed is generated depending on the shape of the inner wall surface of the tubular tissue, because the angle of radiation of the virtual ray is fixed, and the virtual ray is radiated in directions perpendicular to the central path.
FIG. 17 is a view for explaining the problem in the curved cylindrical projection method. As shown in FIG. 17, when the inner wall surface of the colon 121 is observed, it is difficult to observe the backside (see the arrow p) of a fold of the colon 121, because virtual ray 123 is radiated in directions perpendicular to the central path 122.
In “Virtual Colon Unfolding”, A. Vilanova Bartroli, R. Wegenkittl, A. Konig and E. Groller, IEEE Visualization, U.S., pp. 411-420, 2001, above problem is tried to be solved by a method in which a folded structure of a surface of the target internal organ is unfolded by an approach of finite-element deformation after obtaining the shape of the surface of the target internal organ. However, it is difficult to say that this method is practical, because this method has disadvantages such as that subjective and complex condition setting is necessary in the extraction of the surface of the internal organ, and in the process of unfolding, lesion can not be detected because polyp is also unfolded, and calculation for extracting and unfolding the surface of the internal organ is enormous.